function [cust_U] = custom_convection(xx,t)
    %return cust_U with the value of the convection velocity "U" for each node
    %note that cust_U has as many columns as dimensions number of the
    %problem
    [nnod,ndm] = size(xx);
    cust_U = zeros(nnod,ndm);
    %% completar desde aqui
end